The present invention relates generally to cable-cutting devices adapted for use with low-flying aircraft such as cropdusters, helicopters, or the like. More specifically, the present invention relates to an active, automatic-fire, cable cutting device adapted to be associated with an aircraft to prevent damage typically resulting from entanglement with above-ground cables. It is believed that the invention is properly classified in U.S. Class 244, subclass 121.
As will be appreciated by experienced pilots, the safe operation of low-flying aircraft used, for example, for agricultural applications or tactical low-level (NOE) maneuvers, is deleteriously complicated by the existence of numerous above-ground power lines or cables. My investigations reveal that approximately eight percent of commercial helicopter accidents occur as a result of collisions with conventional power cables, while up to thirty percent of reported agricultural collisions are caused by cable strikes. Further, thirty-five percent of military aircraft are grounded by cable strikes occurring during NOE flights.
Helicopters are particularly vulnerable to damage from such cables since the rotor mast extends up a significant distance beyond the airframe structure of the aircraft, and critical control mechanisms for such aircraft are operationally associated with the rotor mast. Approximately fifty percent of all aeromedical helicopter accidents, for example, are attributed to such cable strikes, which in 1985 and 1986 accounted for an average of twelve injuries per one hundred thousand flight hours.
A low-flying aircraft which strikes a typical cable in tension may, on occasion, break the cable by the mere force of its impact and thus escape damage. However, conventional power cables are typically strung with a predetermined amount of slack to prevent breakage due to wire fatigue or excessive vibration in response to air turbulence.
By way of example, a one-inch cable strung 1,150 feet between vertical support structures typically exhibits forty five feet of slack. If an OH6 light military helicopter struck such a cable at a speed of 50 knots, and the cable was not immediately broken or cut, the aircraft would be dramatically decelerated to an abrupt halt (zero airspeed) before the aircraft advanced sufficiently to remove all the slack in the cable. If the cable is not broken upon impact, as is often the case with conventional cable cutters, it may cause the aircraft to change direction and become entangled in additional surrounding cables or drag the aircraft to the ground. Additionally, surrounding cables may become entangled in the rotor blades or about the rotor mast and thus cause extensive damage to the aircraft control systems.
Hence it is desirable to provide low-flying aircraft with a system for removing above-ground cables which threaten the safe operation of the aircraft. In the prior art, safety devices for the deflection or cutting of such cables have been proposed. The prior art cable cutting device disclosed in U.S. Pat. No. 4,215,833 issued to Chan on Aug. 5, 1980 includes passive cable deflection means for guiding a cable between adjacent cutting edges to effectuate at least a partial cut which arguably causes the cable to fail under tension. The effectiveness of the prior art devices defined in U.S. Pat. No. 4,215,833 and in each of the prior art cutters referenced therein, however, is based entirely on kinetic energy, as expressed in the acceleration of the aircraft, and is thus limited to a narrow range of tension load and aircraft speed variables.
Known prior art systems fail where, for example, the aircraft strikes the cable at an insufficient rate of speed and/or is unable to "drag out" the slack in the cable as described above to achieve sufficient tension to effectuate a cut. If the initial impact of the cable strike breaks insulators on the power lines, the cable must be dragged an additional length to attain the tension required for a cut. No auxiliary cutting means are believed suggested in the prior art for such a case. This is particularly critical, since it is often necessary to fly at low speeds to maneuver under low visibility conditions such as may be encountered during severe weather, during night operations, or with terrain avoidance procedures.
A further shortcoming of known prior art devices is that effective use often requires that the airframe be equipped with additional reinforcing structure in order to withstand the loads transferred directly to the airframe. Of limited relevance in this regard in U.S. Pat. No. 3,362,289, issued to Guin on Jan. 9, 1968, which suggests the possible advantage of employing an extension cable or pole for operating aircraft weapons at ground level. However, the latter reference also teaches that such extensions also necessitate the addition of reinforcing and/or bracing structure.
No provision is made in the known prior art for fast-action, successive cutting where, for example, multiple cables are encountered. Chan, U.S. Pat. No. 4,215,833, expressly rejects the concept of providing active, explosion-activated cable-cutting means alleging that such devices are impractical and unsuitable because of excessive weight, complexity, and cost. Even if speed and impact force are sufficient to accomplish a first cable cut, so much acceleration would be lost in accomplishing the first cut that it would be virtually impossible for known prior art devices to properly effectuate a subsequent cut.
It is thus desirable to overcome the previously encountered difficulties and present an active cable cutting system which has been proven to be effective under a broad range of conditions, which can be effectively and economically produced, and which can be easily adapted for use with a variety of conventional aircraft. It would further seem advantageous to provide an aircraft cable cutting system designed to make optimum use of both kinetic and potential energy adapted to operate independently of variables such as airspeed, wire tension, or aircraft structural strength.